REVIEW ARTICLE Concepts on functional appliances’ mode of action Carine Carels, D.D.S., Ph.D.,* and Frans P. G. M. van der Linden, D.D.S., Ph.D.** Nymegen, The Netherlands Several working hypotheses have been conceived to explain the mode of action of various functional appliances. A review of the different concepts is presented and these concepts are tested by means of data from basic scientific research. Furthermore, a general concept is postulated that could unite the working principles of several functional appliances. Finally, the clinical relevance of scientific research in this area is evaluated. (AM J ORTHOD DENTCIFAG ORTHOP 1987;92:162-8.) I n the last 20 years, functional appliance therapy has become a generally accepted method to treat severe and moderate discrepancies of sagittal jaw re- lations in children. Until now, functional appliance ther- apy had its greatest application and success in Class II malocclusions. A variety of different functional appliances is avail- able.’ The appliance selected for the treatment can be adapted to the type of anomaly and to the growth pat- tern. The growth direction, the growth amount, and the timing are relevant to the ultimate success of the treat- ment. Consequently, diagnosis and case selection are critical for functional treatment. Each proponent of the different functional appli- ances has conceived more or less his own concept and working hypothesis: Andresen, Haupl, and Petrik,’ Herren and Harvold4 for the activator; Baiter? for the Bionator; FAnke16 for the functional regulator; Stockfisch’ for the Kinetor; and Bimle? for the Ge- bissformer. This also applies to Pancherz’ for the Herbst appliance and to those who advocate the combination of functional appliances with extraoral appliances: Pfeiffer and Grobety” for an activator with a cervical headgear; Teuscher,” and Lehman and van Beck” for the combination of an activator-type appliance with a high-pull headgear; van Beek13 for the activator-head- gear; BassI for a Frankel-type appliance with a high- pull headgear; and Wieslander” for a combination of the Herbst appliance with a high-pull headgear. It is the aim of this article (1) to present a review of the different concepts on functional appliance ther- From the Department of Orthodontics, University of Nymegen. *Research Assistant of the National Fund for Scientific Research, Department of Orthodontics, Leuven, Belgium; Trainee, Department of Orthodontics, Ny- megen, The Netherlands. **Professor and Chairman, Department of Orthodontics, University of Ny- megen, The Netherlands. 162 apy, (2) to put these concepts to test by means of data from basic scientific research, and (3) to postulate a general concept, based on the first author’s research, that could unite the working principles of several func- tional appliances.